Conventionally, there is known a plug for high frequency emission provided at one end of a casing thereof with an emission antenna. For example, Japanese Unexamined Patent Application, Publication No. 1983-213120 discloses a glow plug attached to a diesel engine as this kind of a plug for high frequency emission.
The glow plug disclosed in the Japanese Unexamined Patent Application, Publication No. 1983-213120 includes an outer conductor in the form of a tube-like shape, an inner conductor passing through an axial center of the outer conductor, a resistance wire connected to the outer conductor and the inner conductor respectively in a substantially integrated manner, and a dielectric filled between the outer conductor and the inner conductor. The outer conductor is formed, at an outer peripheral part thereof, with a thread for attachment to a cylinder head. The resistance wire is protruded toward the inside of a combustion chamber and formed to be a loop-like shaped antenna for microwave emission.